add SetDecorations action

[xi]

as discussed in IRC. resolves #1728

We also discussed adding a "decorations" query for the if-action. I will work on that separately, but this already lays some ground work (e.g. view_get_ssd_mode).

[johanmalm]

I appreciate the work on this and noted the lengthy analysis. I haven’t been able to keep up with the story/thinking so if you want me to review anything, please could you point me to it.

If @ahesford or @Consolatis are already on the case I’m happy not to duplicate effort, but as I said if independent view is required (by me) please make it clear.

[johanmalm]

Thanks for working on this. Sorry it took a while to get into the merge window.